I remember seeing a post about this, though I can't remember whether it was here - I couldn't find it, efficiently, nonetheless.
In case anyone has trouble keeping the bung/stopper in the top of glass demijohns/carboys, I made simple muselets out of 16 AWG craft wire.
How-To images follow the 'deployment' images.
Fermentor (apologies for the poor lighting, but you know, fermentation chamber.
Aging a mead, or any secondary.
HOW TO MAKE IT:
1. Cut craft wire, approximately 2 feet, folded in half.
2. Twist a loop into end - at middle fold - a few times. I do this so that the loop will end up just below the top of the bung.
3. Place wire about demijohn neck, where you will want it to set (has to be under a lip, obviously).
4. Wrap around and CAREFULLY pull the wire tight (should do this while demijohn is empty, just in case).
5. Twist pair to close.
6. Place bung. Loop ONE of the wire ends over the bung, slightly off-center, and thread through the loop.
7. CAREFULLY pull the wire tight, and kink the pair to set.
8. Lay second wire end over the first and the loop.
9. Pull second wire-end tight, and twist to set.
ALL DONE!!!
You can see there's plenty of room for an airlock. I normally only use the drilled stoppers, and not the type used in this demo - they are WAY more reliable, in my experience.